Heat Transfer Coefficients
It is often useful to determine values for overall heat transfer coefficients
while performing non-exact activities such as early project cost estimating and basic heat exchanger performance assessments.
The equation which relates the overall heat transfer coefficient to the heat duty
and the heat transfer area is:
Q= U*A*DTlm
Where:
Q = heat load
U
= overall heat transfer coefficient
A = heat transfer area
DTlm = log mean temperature
difference
Overall heat transfer coefficients are dependant on many parameters such as the
nature of the fluid, fluid velocities, type of heat exchanger, temperatures and fouling. Despite all these determining parameters,
typical overall heat transfer coefficients are available for common applications and fluids. If little information about the process and the parameters
outlined above is available, the following values can be used as a guide for overall heat transfer coefficients:
Sensible
Vapour:
30 Btu/hr-ft2-F
Sensible Heating/Cooling or Condensing: 100
Btu/hr-ft2-F
Boiling: 120
Btu/hr-ft2-F
When more information about the fluids and process is available, one can use the overall heat transfer
coefficient values in the tables below as a guide as to the order of magnitude. Actual overall heat transfer coefficients may be smaller or larger than the values
listed.
Heaters (no phase change) |
| Hot Fluid | Cold Fluid | Overall
U
(BTU/hr-ft2-F) |
| Steam | Air | 10
– 20 |
| Steam | Water | 250
– 750 |
| Steam | Methanol | 200
– 700 |
| Steam | Ammonia | 200
– 700 |
| Steam | Aqueous
solutions | 100 – 700 |
| Steam | Light hydrocarbons
(viscosity < 0.5
cP) | 100 – 200 |
| Steam | Medium hydrocarbons
(0.5 cP < viscosity < 1 cP) | 50
– 100 |
| Steam | Heavy
hydrocarbons
(viscosity > 1) | 6 – 60 |
| Steam | Gases | 5 – 50 |
| Dowtherm | Gases | 4 – 40 |
| Dowtherm | Heavy oils | 8 – 60 |
| Flue gas | Aromatic hydrocarbon and steam | 5
– 10 |
Evaporators |
| Hot
Fluid | Cold Fluid | Overall U
(BTU/hr-ft2-F) |
| Steam | Water | 350 –
750 |
| Steam | Organic
solvents | 100 – 200 |
| Steam | Light
oils | 80
– 180 |
| Steam | Heavy
oils (vacuum) | 25 – 75 |
| Water | Refrigerant | 75
– 150 |
| Organic solvents | Refrigerant | 30
– 100 |
Coolers (no phase change) |
| Cold Fluid | Hot Fluid | Overall
U
(BTU/hr-ft2-F) |
| Water | Water | 150
– 300 |
| Water | Organic
solvent | 50 – 150 |
| Water | Gases | 3
– 50 |
| Water | Light
oils | 60
– 160 |
| Water | Heavy
oils | 10
– 50 |
| Light oil | Organic
solvent | 20 – 70 |
| Brine | Water | 100
– 200 |
| Brine | Organic
solvent | 30 – 90 |
| Brine | Gases | 3
– 50 |
| Organic solvents | Organic
solvents | 20 – 60 |
| Heavy
oils | Heavy
oils | 8
– 50 |
Condensers |
| Cold
Fluid | Hot Fluid | Overall U
(BTU/hr-ft2-F) |
| Water | Steam (pressure) | 350 -750 |
| Water | Steam (vacuum) | 300 – 600 |
| Water or brine | Organic solvent (saturated, atmospheric) | 100
– 200 |
| Water or brine | Organic solvent (atmospheric, high non-condensables) | 20
– 80 |
| Water or brine | Organic solvent (saturated, vacuum) | 50 – 120 |
| Water or brine | Organic solvent (vacuum, high non-condensables) | 10 – 50 |
| Water or brine | Aromatic vapours
(atmospheric with non-condensables) | 5
– 30 |
| Water | Low
boiling hydrocarbon (atmospheric) | 80 – 200 |
| Water | High boiling hydrocarbon (vacuum) | 10 – 30 |
When the process is well defined, one can use film heat transfer coefficients
to calculate the overall heat transfer coefficient.
The overall heat transfer coefficient
can be calculated from the film coefficients using the equation:
1 = 1
+ Rout + Rwo +
Rio + 1
U hout
hio
Where:
U = overall
heat transfer coefficient
hout = film coefficient on outside surface
Rout
= resistance due to fouling on outside surface
Rwo = resistance due
to metal wall of heat transfer area (corrected to the outside)
Rio = resistance due to fouling on inside surface (corrected to the outside)
hio = = film coefficient on inside surface (corrected to the outside)
In order to use the equation above, values for the film heat transfer coefficients
must be determined. Film coefficients, just like overall coefficients, are influenced by many parameters such as nature of the fluid, type of heat exchanger,
fluid velocity, transport properties and temperature. The tables below provide examples of film coefficients values for various applications.
Again, these should be used as a guide as to the order of magnitude and the actual film coefficients may be smaller or larger than the values listed.
no
phase change |
| Fluid | Film
Coefficient
(BTU/hr-ft2-F) |
| Water | 300
– 2000 |
| Gases | 3
– 50 |
| Organic Solvents | 60
– 500 |
| Oils | 10
– 120 |
Condensing |
| Fluid | Film
Coefficient
(BTU/hr-ft2-F) |
| Steam | 1000
– 3000 |
| Organic Solvents | 150
– 500 |
| Light Oils | 200
– 400 |
| Heavy Oils (vacuum) | 20
– 50 |
| Ammonia | 500
– 1000 |
Evaporation |
| Fluid | Film
Coefficient
(BTU/hr-ft2-F) |
| Water | 800
– 2000 |
| Organic Solvents | 100
– 300 |
| Light Oils | 150
– 300 |
| Heavy Oils | 10
– 50 |
| Ammonia | 200
– 400 |
The information is provided for educational use only – use at your own risks.